Monotube boiler feedwater and steam temperature control



Jan. 5, 1965 H. VON BECK 3,154,135

MONOTUBE BOILER FEEDWATER AND STEAM TEMPERATURE CONTROL FIG. I

ABSOLUTE PRESSURE BIA.

7 11 I20 m;%!"&$ Q 2 k a 8 r f wxx 400 x 0, A)

0 \0 14/! 600 710i V a l 7200 RH -g,i%-g 7-= 70ar a 0/0 205 g 5 U #600 l E l 0 pl 5 5 I 5200 5600 4000 4400 4000 5200 i L INVENTOR. 200 HAROLD VON BECK BY @0027 0 400 000 /200 /000 2000 i 2000 2000 AGENT H. VON BECK MONOTUBE BOILER FEEDWATER AND STEAM TEMPERATURE CONTROL Filed Jan. 27, 1961 2 Sheets-Sheet 2 m m %N m m INVENTOR. HAROLD VON BECK $4. a

AGENT ,rstass Patented Jan. 5,1965

United States Pater Q The invention relates to a once-through vapor generator operating in the supercritical pressure range and to a method for controlling the operation thereof. More particularly the invention is concerned withan improved method and an improved arrangement of devices for controlling the feedwater flow to a once-through steam generator and for controlling the temperature of the superheated steam produced at a pressure above the critical pressure, 3,206.2 p.s.i.a. solute); p 7 p In a steam generator equipped with a steam and water drum a relatively large reservoir of water is generally (pounds per square inch abavailable. When operating a boiler ofthis type with hatural circulation ah increase in firing rate produces an 1n-- crease in circulation of the water through the heated tubes. a pump is provided to give adequate water flow for maximum heat absorption in the boiler. T 'In a once-through steam generator however the water" flow through the heated tubes only depends on the feedwater quantity entering the boiler'which must be regulated toinatch the firing rate or heat input. V

In one commonly adapted control system the feedwater flow as well as the temperature of the superheated steam is regulated in response to a steam temperature impulses. To overcome the undesirable lag in response experienced with steam temperature control, especially during sudden load changes, it has been found desirable to make use of anticipating control signals such as impulses of steam flow." v Based on experience with boilers operating in the subcritical pressure range, a steam flow indicator such as an orifice has been employed for anticipating fluctuations in Y heat input also in boilers operating in the supercritical pressure range. However such control has proven to be unsatisfactory because it is 'too strongly influenced by other factors such as feedwater flow.

It is accordingly a primary object of the invention to anticipating impulses which are quick acting and sensitive, and provide a useful index of heat absorption that is free of the influence of outside factors.

Other objects and advantages of the invention will become apparent from the following description of an illustrative embodiment thereof when takenin conjunction with the accompanying drawings wherein:

In a boiler operating with controlled circulation,

. reflected in the steam into conformity with the firing rate so as t a I by-way of-fuel burner 12 and air inlet duct 14. The Walls of chamber iii are lined with water carrying and steam generating tubes which in the diagrammatic illustration of FIG. 1 are represented by tubes 16 terminating in an inlet 18 and an outlet Ztl. In passing through chamber it) the hot combustion gases give up heat to steam generating tube 16,.to a superheater Hand to an economizcr 24. Other conventional heat absorbing surfaces such as a steam reheater or an air heater, not shown, may be placed for absorption of heat in the path of the combustion gases on their way to a stack, not shown.

In operating the steam generator economizer 24 receives feedwater of a relatively low temperature by way ofv a feed pump 26 and feed pipe 28. After heating the water-in the economizer 24 to a suitable temperature, the water fiows via pipe 3t) and inlet 13 into water Wall tubes 16 for further heating and generation of steam. After passing through outlet 2% and conduit 31, additional heating of this steam to a predetermined desired temperature takes place in superheater ZZfrom whence v the steam is delivered by way'of steam pipe 32 to a point of use such as steam turbine 34 driving an electric generator 36. Having given upits energy by expansion in turbine 34 "the steam or condensate is-exhausted therefrom by way of conduit 33 into condenser d0 fromwhence the condensate is delivered-to the inlet of feed pump 26 by way of, conduit 41 thereby completing the. oncethrough steam producing cycle. Control of the generation of steam may bev acomplished by varying the firing rate in response to turbine load demand or othersystem load requirements. Variations in steam demand will be pressure prevailing in steam piping 32 as indicated by steam gage 42. A momentary drop in steam, pressure due to a greater demand of steam will cause fuel regulating device such as fuel valve or fuel feeder 44 and air damper 46 to admit more fuel and air into chamber 10. Conversely a rise in steam pressure will cause a diminishing of fuel and air flow into chamber 10 in a manner well known in the art. Having adjusted the fuel and air supply to the load demand in this manher, the nextcontrol step isto bringthe feedwater flow to maintain a suflicient feedwater supply and a constant pressure and temperature of the'steam regardless of load variations.

In accordance with the invention as illustratively shown in FIG. 1, this is accomplished by extracting a relatively small amount of working fluid at supercritical pressure FIG. 1 is a diagram representing a feedwater control system for a once-through steam generator operating in the supercritical pressure range and employing the novel feedwater control system and method disclosed by the invention.

FIG. 2 is a conventional enthalpy diagram illustrating the relationship of the enthalpy values of water and steam and'absolute pressures thereof for various temperature parameters. i

FIG. 3 is a diagram representing a control system similar to that shown in FIG. 1 with additional control impulses received for regulating the superheated steam ternperature.

In a once-through steam boiler as diagrammaticallyi range, the border illustrated in FIG. 1 combustion elements such as fuel and air are introduced into steam generating chamber it) limits (within the area V reduction to 400 p.s.i'.a.

theoretically be feasible.

from the once-through circuit such as at 50, preferably in the region of enthalpy of between 700 to 1100 Btu. per pound (British thermal unit per pound). A pressure reducing device-is provided, here'shown in the form of throttling valve 52, to reduce the pressure from a super.

critical pressure value to. predetermined subcritical pressure.

critical pressure value to a predetermined subcritical I I as shown in FIGQZland while maintaining constant enthalpy results in conversion of the vaporized fluid into a two-phase mixture of liquid and vapor.

The above is illustratively' shown hi the diagramof FIG. 2 wherein steam temperature parameters are plotted against enthalpy v and pressure. where this conversion is preferably initiated lies between enthalpy values of 700 and 1100 Btu. per pound as earlier pointed out, a higher limit, as high as 1200 Btu. per pound would theoretically be feasible with a pressure thalpy range below 700 Btu. per pound would likewise Wrepresents, when operating in the subcritical pressure line between. the liquid region L and the c In accordancevwith thermodynamic laws, re}, ducmg the pressure of a vaporized fluid from a super-.

Although the region 10bviously extending the enln the diagram'of FIG. 2 curve 1 To illustrate the above, if steam is extracted from a supercritical circuit such as at point 50 (FIG. 1), at 4400 p.s.i.a. and at a temperature of 720 F. as indicated by point M in FIG. 2, this vaporized fluid would have an enthalpy of 800 B.t.u. per pound. When reduced to a pressure of 680' p .s.i.a., while theoretically maintaining constant enthalpy as indicated by point N in FIG. 2, the heat in the liquid will amount to 490 B.t.u. per pound and the heat of evaporation to 310 B.t.u. per pound with the total enthalpy in the steam and water mixture being 800 B.t.u. per pound and the total heat of evaporation at this pressure being 710 Btu. per pound. By maintaining the back pressure or subcritical pressureto which the extracted steam is reduced at aconstant value, the relative proportion of vapor to liquid in the mixture accordingly becomes a criterion for the total enthalpy or heat absorbed by the fluid. As will be noted from the enthalpy diagram of FIG. 2 the proportion of steam in the mixture is 310/710 or 43.6 percent and that of water 400/710 @564 percent. A drop in heat absorption caused by a drop in firing rate or other causes as indicated by line U,

r: instance, would entail a lowering of the percentage of steam to 265/710 or 37.3 percent or a reduction of 436/436 minus 37.3/43.6 or 14.5 percent. Thusif in the cited example the reduction in firing rateshould cause a drop in temperature, for instance from 720 F. to 700 F. or about 2.78 percent, a corresponding drop in steam quantity proportion in the mixture when reducing the pressure to 680 p.s.i.a. would occur from 43.6 to 37.3 or 14.5 percent. about 5.3 times as high as the percent variation obtained by temperature measurements (14.5 divided by 2.78).

Or, considering another example, if the steam extracted irom a supercritical circuit at .4400 p.s.i.a. and 720 F.

temperature as indicated by M in FIG. 2 is reduced to pressure of 2220 p.s.i'.a as indicated by N, the heat in the liquid will amount to 700 Btu. per pound, with a total of 800 B.t.u. per pound of mixture and a total heat of evaporation at 2220' p.s.i.a. of 415B.t.u. per pound. As in the earlier example, if a drop in heat absorption would cause a drop in temperature from 720 F. to 700 F. or about 2.78 percent, a corresponding drop in steam quantity when reducing. the pressureto 2220 p.s.i.a., in this case, would occur from 100/415 .to 55/415 or 100/100 minusSS/lOO or .45 percent. stitute a percentage variation about 16.2 times as high as the percent variation obtained by temperature measurements (45 divided by 2.73). I

From the above, it can readily be appreciated that when determiningv the relative quantities of steam and water from the heat in the steam and water mixture that results from reducing the pressureof a supercritical fluid to the subcritical range, an accurate and dependable measure for purposes of feedwater regulation results, vastly superiorto obtaining anticipating impulses by measurements of supercritical fluid temperature. How such superior measurements'ar'e obtained and are applied in accordance,

' with the invention will now be described.

' A pressure sensing device 54, see FIG. 1, is employed in co-action with throttling valve 52 and control valves 56 to maintain a constant back pressure in the steam and .water mixture passing through conduits, 57, 58, 59, and 60. The steam and water mixture leaving valve 52 passes into a separator 62 wherein the steam is separated from the water,'the formerflowing through conduit 59 to a region of lower pressure such as the inl'et'to condenser 4-0. A flow measuring device 64 provided in conduit 59 senses variations in steam flow. These impulses are transmitted to fcedwater regulator 66 for controlling the positioning of feedwater valve 67 and constitute an anticipating-control signal.

This would con- Under certain conditions it may be desirable to supplement the above signal of steam flow measurements by water flow measurements of the water leaving separator 62. Such measuments can be obtained by a flow meter 68 provided in conduit 58 conducting the separated Water to a point of low pressure such as to the outlet of condenser 40. The water flow impulses obtained by means of device 63 may be evaluated in a balancing device 72 against steam flow impulses obtained by device 64, with the object of maintaining a predetermined ratio between steam and water flow with feedwater flow suitably regulated by impulses received from device '72. This arrangement would render the feedwater control less dependent on maintaining a constant boiler pressure. Or variations of the water flow impulses or the steam flow impulses may be employed as they are separately and independently received'by feedwater regulator 66 for regulating the feedwater flow from flow measuring devices 68 or 64 respectively, these impulses being evaluated against predetermined proportional quantities of steam or water, when the temperature of the steam at point 50 is normal.

Thus, if the pressure maintained at the pressure indicating device 54 is 680 p.s.i.a. as in the example earlier discussed herein, the percentage of steam measured by I accordingly constitutes the set point.

This constitutes a percentage variation steam flow indicating device 64- would be 43.6% and that of water 56.4%, with the boiler normally operating at 4400 p.s.i.a. and 720 F. This percentage relationship A decrease of the percentage of steam or an increase of the percentage of water therefore would indicate a feedwater quantity in excess of that needed to match the prevailing heat input and would result in regulation of valve 67 such as to redu'cethe feedwater flow. On the other hand, an increase in the percentage of steam or a decrease in the percentage of water below the set point would result in regulation of valve 67 such as to increase the feedwater flow.

From the above it can readilybe understood that the set point depends on the selected subcritical pressure that is maintained in line 57, which pressure determines the relative percentage of steam and water leaving separator 62 for a final steam temperature of 700 F., for example. This set point pressure may be chosen to be the same, such as 680 p.s.i.a., for all loads. Or a different set point such as for example, if the unit is being operated for an extended period at reduced loads.

Another means for evaluating the relative quantities of steam and water in the vapor'and liquid mixture flowing through conduit 57, in accordance with the invention, is a steam quality measuring device 74. This device serves the purpose of determining the quality or wetness of the steam leaving throttling valve 52, such variations in steam quality being a direct indication of the heat release or firing rate. Within a certain pressure range a calorimeter of a design well known in the art could be employedfor this purpose. Other suitable devices could be used such as a recording flame photometer determining the wetness of the steam by measuring variations of sodium content in wetness of the steam can be measured directly, the neces-' sity of first separating the steam phase from the water phase in separator 62 is dispensed with.

I The above-described pre-adjusting or anticipating control impulses received by the feedwater regulator 66 from steam flow indicator 64, water flow indicator 68 or steam quality indicator 74 are then supplemented by final impulses, obtained fromsteam temperature variations at location 50, or other convenient location, as they are sensed by temperature indicating device '76, and transmitted by impulse transmitting conduit 78 to regulator 66. Thus feedwater in a supercritical pressure steam generator is controlled by combining final impulses of steam temperature with anticipating impulses received from the characteristics of a steam and water mixture in the subcritical pressure range as hereinabove described. This will produce a control system for feedwater regulation that is both dependable, quick acting and substantially free of hunting tendencies.

. 6; tails set forth but desire to avail myself of such changes as fall within the purview of my invention.

I claim:

1. In a fluid heating apparatus having circuit means producing throughout the normal operating load range thereof a vapor in the supercritical pressure range, in

combination, means for controlling the rated quantity'of' the fluid fed to said circuit, means for heating said fluid to a temperature in excess of the critical temperature and at a pressure in excess of the critical pressure, means for extracting throughout said normal operating load range a portion'of said fluid from said circuit said por- These improved anticipating controlsignals, however,

may also be applied in a control circuit concerned with maintaining the temperature of the steam at a constant value with variations in steam load. This system is illustratively shown in FIG. 3 and will now be described.

- One preferred method of controlling the temperature of the superheated steam entering steam turbine 34 is by injecting a controlled quantity of feedwater into the flow path of superheated steam at one or more points such as at point 80 (FIG. 3). This is accomplished by.

extracting feedwater from the feed line 28 and conducting it to the injection point 80 by-way of pipesl. A valve 82 controls the amount of water thus injected for reducing the steam temperature to a desired. value.

. Valve 82 is actuated. by an injection Water controllerobtained from the supercritical pressure steam extracted 1 at point 50 as hereinabove described in connection with FIGS. 1 and 2. 'Under certain operating conditions it may be desirable to only employ characteristics of the,

steam and watermixture as anticipating signal for steam temperature control.

Thus the heat content of the supercritical pressure fluid at point-5t} furnishes a pre-adjusting control via the subcritic'al pressure mixture-of steam and water obtained in pipe 57, both for regulating the feedwater flow via feedwater regulator 66' as well as for maintaining a constant steam temperature via injection water controller 83. I

From the above it can be appreciated that the hereindisclosed system for controlling the operation of a steam boiler in the supercritical pressure range not only retains" all the advantageous'features of a control system fora steam boiler operating inthe subcritical pressure range" but establishes a much desired closer relationship between feedwater controliyand steam temperature .control by the use of a common anticipating control impulse via controller 66. According to the invention this is basically accomplished by extracting a portion of the supercritical pressure fluid the desired enthalpyof'which is known, reducing the pressure thereof to a predetermined subcritical pressure therebynproducing a mixture of water and steam, measuringand utilizing thermal characteristics of this mixture that are indica--' tive of variations of the heat content thereofxfrorn the desired or expected heat content, for the purpose of controlling the feedwater flow or steam'temperature or both.

' WhileI have illustrated and described preferred em- "bodiments of my invention, it is to be understood that such are merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. I therefore do not wish to be limited to the precise detion being independent of the load demand, means for reducing the pressure of said extracted fluid portion from said supercritical pressure to a predetermined subcritical pressure to cause said fluid portion to form a twophase mixture of vapor and liquid, means-for separating the vapor from the liquid, means for eflectively determining the actual flow. rateof liquid relative to the actual flow rate of vapor in said mixture, and means for increasing or decreasing respectively the quantity of liquid fluid fed to said circuit as said actual vapor flow 3 rate increases and said actual liquid flow rate decreases,

or as said actual vapor flow rate decreases and said actual liquid flow rate increases. K I

2. In a fluid heating circuit. producing throughout the normal operating load range thereof a vapor in the supercritical pressure range having means for feeding relatively cool liquid fluid to said fluid heating circuit under a pressure in excess of the critical pressure and at an inlet temperature below the critical temperature, said circuit having means for heating. said liquid fluid to'an outlet temperature in excess of said critical temperature-for the production of vapor from said fluid, the combination ofmeans for controlling the flow of said fluid fed to said heating circuit, means for extracting throughout said normal'operating load range a -portion' of said. fluid from said circuit at a location that yieldsfluid atfa temperature inexcess of said critical temperature said portion being independent of the load demand,-flow restricting means for throttling the flow of said extracted portion and for reducing the supercritical pressurethereof to a predetermined subcritical pressure to cause said fluid to form a two-phase mixture of vapor and liquid, means for eflectively determining the quantity of fluid of at least 1 one of said phases while'maintaining saidpredetermin'ed subcritical pressure, and meansfor adjusting said flow controlling means for regulatingsaid cool fluid flow in phase. I V i 3. In a fluid heating circuit; producing throughout the normal operating load range thereof a vapor in the supercritical'pressure range having means for feeding relatively cool fiuid'in the liquid phase to said fluid heating cirresponse to variations of the fluid quantity in saidone cuit under a pressure in excess of the critical pressure and at an inlet temperature below the critical temperature, said circuit having means for heating said liquid fluid to an outlet temperature in excess of said critical tem perature for the productionof vapor from said fluid,

the combinationof meansfor controlling the flow of said liquid fluid fed to said heating circuit, means for extracting throughout said normaloperating load range a portion of said fluid from said circuit at a location that yields fluid at a temperature in excess of said critical temperature said portion being independent of theload demand, flow restricting means for throttling the flow of said extracted portion and for reducingthe pressure thereof to a predetermined pressure below the critical; pressure to cause said extracted'fluid to form a two-- phase mixture of liquid'and vapor, means for obtaining an indication of the fluid. quantity of at least one of said phases while maintaining said predetennenid pres-; sure, and means for'adjusting said cool fluid flow cona 7- 1 trolling means in response to variations in said fiuid quantityiindication in-said one' of said phases. r I

4. In a fluid heating circuit producing throughout the normal operating load range thereof a vapor in the supercritical pressure range having means'for feeding relatively cool fluid in the liquid phase to said fluid heating circuit under a pressure in excess of the critical pressure and at an inlet temperature below the critical temperature, said circuit having'means "for heatingrsaid liquid fluid to an outlet temperature in excess of said critical temperature for the production of vapor from said fluid, the combination of means for controlling the flow of said liquid 1 fluid fed to said heating circuit, means for extracting throughout said normal operating load range a portion of said fluid from said circuit at a loction'that yields fluid at a temperaturein excess of said critical temperature said portion beingindependent of the load demand, flow restrictingmeans for throttling the flow of said extracted portion and for reducing the pressure thereof to a pre-' determined'pressure below the critical pressure to cause said extracted fluid to form a two-phase mixture of liquid and vapor, means for separating said phases and for effectively determining an indication of the rated fluid quantity of at least one of said phases while maintaining said predetermined pressure, and means for adjusting said flow controlling means'to respectively increase or decrease said cool liquid fluid flow in response to an increase or decrease of the ifluid quantity indication of'said one phase. r I 7 M 5. In ajvaporigenerator having aonce-through fluid passage receiving liquid at one end and delivering throughout the normal load range thereofa superheated vapor atsupercritical pressure and under normal operating conditions at the other end, and elements of combustion for -hcating said fluid, the combination ofmeans for extracting throughout said normal operating load range a portion of said fluidfrom said fluid passage at a point thereof yielding fluid at supercriticalpressure and of supercritical temperature said portion being independent of the load demand, meansfo'r reducing the pressure of said extracted fluid to a predetermined subcritical pressure to form a two-phase mixture of liquid and vapor, means for separating the vapor from, the liquid, means for obtaining indications of the: flow rates of said liquid and vapor and means for. regulating the flow rate of liquid received at said one'pressure end in response to deviations pressure to a predetermined pressurebelow the critical pressure to cause said fluid portion to form a two-phase mixture of liquid and vapor, separating the vapor from the liquid, eflectively determining the fluid flow rate in at least one of said phases While maintaining said predetermined pressure, and adjusing said unheated flow rate in response to variations in the fluid flow rate in one of said phases.

8. In a fluid heating circuit producing throughout the normal operating load range thereof a vapor in the supercritical pressure range, the method of conrolling the unheated fluid fed to said circuit, comprising the steps of heating said fluid to an exit temperature in excess of the critical temperature and at a pressure in excess of the critical pressure, extracting throughout said normal operating load range a portion of said fluid and said portion being independent of the load demand reducing the pressure thereof from supercritical pressure to a predetermined subcritical pressure to cause said fluid portion to form a two-phase mixture of liquid and vapor, separating the vapor from the liquid, effectively determining the fluid flow rate in at least one of said phases while maintaining said predetermined subcritzical pressure, and adjusting said unheated flow rate in'responsc to variations in the fluid flow rate in said one phase.

9. In a fluid heating circuit producing throughout the normal operating load range thereof a vapor in the supercritical pressure range, the method of controlling the fluid fed to said circuit, comprising the steps of heating saiid fluid to an exit temperature in excess of the critical temperature and at a pressure in excess of the critical pressure, extracting throughout said normal operating load range a portion of said fluid said portion being independent of the load demand and reducing the pressure thereof from "apressure in excess of the critical pressure to a predetermined pressure below the critical pressure to I cause said fluid portion to form a two-phase mixture of liquid and vapor, measuring the actual ratio of said liquid and vapor and regulating the quantity of fluid fed to said circuit in response to deviations of said actually measured liquid and vapor ratio from a predetermined normal ratio while maintaining said predetermined pressure.

10. In a fluid heating circuit producing throughout the normal operating load range thereof a vapor in the supercritical pressurerange, the method of controlling the fluid fed to said circuit, comprising the steps of heating said fluid to an exit temperature in excess of the critical temperature and at a pressure in excess of the critical pressure, extracting throughout said normal operating loadrange a portion of said fluid said portion being independent of the load demand and-reducing the pressure thereof from a pressure in excess, of the critical pres- 7 sure to a predetermined pressure below the critical presvalue of less than 12% Bin per pound saidquantity being independent of the load demand, means for reducing the pressure of said extracted quantity to a predetermined pressure in the subcritical range to cause the formation of atwo-phase mixture of liquid and vapor; means for measuring the value of a property of said mixture that is indicative of the heat of evaporation of said mixtnre, and means effective in maintaining said superheated vapor at 'aconstant temperature in response to said measuring means. a

7. In a fluid heating circuit producing throughout the heating said 'fluid' to an exit temperature in excess of the critical temperature and at a pressure in excess of the critical pressure, extracting throughout saidnormal operating load range a portion of said fluid, said portion being independent of thc load demand and reducing the pressure thereof'from a pressure in excess of the critical sureto cause said fluid portion to changeinto a .fluid quantity in the vapor state and a fluid quantityiin the liquid state, separating the vapor from the liquid, and increasing or decreasing respectively the flow rate of fluid fed to said circuit as the rate of said vapor quantity increases or the rate of said liquid quantity-increases.

11. In a fluid heating circuit producing throughout the normal operating load range thereof a vapor in the supercritical pressure range, the method ofcontrolling the fluid fed to said circuit, comprising the steps of heating said fluid to an exit temperaure in excess of the critical temperature and at a pressure inexcess of the critical pressure, extracting throughout said normal operating load range a portion of said fluid said portion being independent of the load demand and reducing the pressure thereof from a pressure in excess of the critical pressure to a cause said fluid portion to change into a fluid quantity in thevapor state and a fluid quantity in the liquid state,

measuring the quantity of fluid in at least one of said states and decreasing or increasing respectively the flow rate of fluid to said circuit as the quantity of said one of said states varies.

12. In a fluid heating circuit producing throughout the normal operating load range thereof a vapor in the supercritical pressure range, the method of controlling the fluid fed to said circuit, comprising the steps of heating said fluid to an exit temperature in excess of the critical temperature and at a pressure in excess of the critical pressure, extracting throughout said normal operating load range a portion of said fluid said portion being independent of the load demand and reducing the pressure thereof from a pressure in excess of the critical pressure to a predetermined pressure below the critical pressure to cause said fluid portion to change into a fluid quantity in the vapor state and a complementary fluid quantity in the liquid state, determining the quantity of vapor relative to the quantity of liquid and increasing or decreasing respectively the flow rate of fluid fed to said circuit as said quantity of vapor increases and said quantity of liquid decreases, or as said quantity of vapor decreases and said quantity of liquid increases.

13. The method of controlling throughout the normal operating load range thereof the operation of a vapor generator having a once-through fluid passage receiving liquid at one end and delivering superheated vapor at supercritical pressure at the other end and heated by elements of combustion, which includes the steps of extracting throughout said normal operating load range a quantity of fluid from said fluid passage at a point thereof yielding fluid at supercritical pressure and of supercritical temperature said quantity being independent of the load demand, reducing the pressure of said extracted fluid to a predetermined subcritical pressure to form a two-phase out said normal operating load range a quantity of said working fluid having an enthalpy value of less than 1200 Btu. per pound said quantity being independent of the load demand, reducing the pressure of said extracted fluid to a pressure in the subcritical pressure range to cause the formation of a two-phase mixture of liquid and vapor, measuring the value of a property of said mixture that is indicative of the heat of evaporation of said mixture, and employing a control impulse responsive to the'property measurement for actuating control means eltective in maintaining said superheated vapor at said constant temeprature.

References Cited in the file of this patent UNITED STATES PATENTS 2,170,346 Dickey Aug. 22, 1939 2,900,792 Buri Aug. 25, 1959 FOREIGN PATENTS 816,765 Great Britain July 15, 1959 817,121 Great Britain July 22, 1959 UNITED STATES PATENT OFFICE QERTIFICATE OF CORRECTION Patent No, 3,164,135 January 5, 1965 Harold Von Beck It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, lines 56 and 57, for "value to a-predetermined subcritical limits" read to a subcritical pressure within certain limits column 5, line 3, for "76" read 76T line 30, for "84" read 84T line 31, for "85" read 8ST column 7, line 46, for "pressure" read passage Signed and sealed this 6th. day of July 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER A 1 testing Officer Commissioner of Patents 

7. IN A FLUID HEATING CIRCUIT PRODUCING THROUGHOUT THE NORMAL OPERATING LOAD RANGE THEREOF A VAPOR IN THE SUPERCRITICAL PRESSURE RANGE, THE METHOD OF CONTROLLING THE UNHEATED FLUID FED TO SAID CIRCUIT, COMPRISING THE STEPS OF HEATING SAID FLUID TO AN EXIT TEMPERATURE IN EXCESS OF THE CRITICAL TEMPERATURE AND AT A PRESSURE IN EXCESS OF THE CRITICAL PRESSURE, EXTRACTING THROUGHOUT SAID NORMAL OPERATING LOAD RANGE A PORTION OF SAID FLUID, SAID PORTION BEING INDEPENDENT OF THE LOAD DEMAND AND REDUCING THE PRESSURE THEREOF FROM A PRESSURE IN EXCESS OF THE CRITICAL PRESSURE TO A PREDETERMINED PRESSURE BELOW THE CRITICAL PRESSURE OF CAUSE SAID FLUID PORTION TO FORM A TWO-PHASE MIXTURE OF LIQUID AND VAPOR, SEPARATING THE VAPOR FROM THE LIQUID, EFFECTICELY DETERMINING THE FLUID FLOW RATE IN AT LEAST ONE OF SAID PHASES WHILE MAINTAINING SAID PREDETERMINED PRESSURE, AND ADJUSTING SAI UNHEATED FLOW RATE IN RESPONSE TO VARIATIONS IN THE FLUID FLOW RATE IN ONE OF SAID PHASES. 